1. Field of the Invention
The present invention generally relates to a control apparatus, and specifically relates to a control apparatus for controlling a gas turbine.
2. Description of the Related Art
A typical gas turbine includes a compressor, a combustor, and a turbine. Air is taken into the gas turbine through an air inlet and the air is compressed by the compressor to produce compressed air that is at high temperature and high pressure. In the combustor, the compressed air is mixed with fuel, and the mixture is burned to produce combustion gas that is at high temperature and high pressure. The combustion gas is used to drive the turbine that in turn drives a generator.
A typical turbine has a turbine casing and a plurality of stator vanes and rotor vanes arranged alternately in the turbine casing on a turbine shaft. The turbine shaft is connected to the generator, so that the generator is driven when the turbine shaft rotates. The turbine shaft is driven and rotated by rotating the rotor vanes by the force of the combustion gas. The combustion gas, after driving the turbine, is converted to gas that is at a static pressure by a diffuser provided in an exhaust casing. The combustion gas at the static pressure is released into the atmosphere.
The turbine shaft is rotatably supported by bearings. Lubricating oil is supplied to the bearings so that the turbine shaft rotates smoothly in the bearings. Moreover, multi-step seal rings are provided near the bearings so that the lubricating oil does not flow out of the bearings. In addition, seal air, which is extracted from the compressor, is supplied to the seal rings so that the lubricating oil does not leak out of the bearings, and high-temperature gas cannot seep into the bearings.
Japanese Patent Application Laid-open No. 2005-023812 discloses a conventional gas turbine.
In the conventional gas turbine, however, the compressor stops when the operation of the gas turbine is stopped. If the compressor stops, the air taken into the compressor is not compressed, so that there is no compressed air that can be used as the seal air. If there is no seal air, the ambient high-temperature gas can enter inside the bearings, or the lubricating oil can leak from the bearings to the outside. If the ambient high-temperature gas enters inside the bearings, the lubricating oil in the bearings is heated and can get carbonized and solidified. The carbonized lubricating oil can adhere to the sealing surface of the seal rings, which causes vibrations of the turbine shaft.
Thus, there is a need of a technology that inhibits heating of the lubricating oil supplied to the bearings.